Dynamical mechanisms of atmospheric regime behavior are investigated in the context of a quasigeostrophicthree-level T21 model of the wintertime atmospheric circulation over the Northern Hemisphere.The model, driven by realistic orography and using a thermal forcing determined by a newly developedtuning procedure, is shown to possess a reasonable climatology and to simulate the Arctic Oscillation quiterealistically. It exhibits pronounced internally generated interannual and decadal variability and, in particular,circulation regimes that agree fairly well with observed ones. Two known hypotheses about theorigin of regime behavior, as it occurs in the model herein are addressed: (i) multiple equilibria and (ii)chaotic itinerancy between attractor ruins. The first hypothesis is falsified at very high probability, while thesecond is likely to be true.